This invention relates to a starting system for a brushless motor and, more particularly, to a starting system for a brushless motor which reduces the pulsations or torque at the time of the start.
The method of starting a DC brushless motor, in which current passes through the individual windings in the brushless motor for a period of 120 degrees is well known in the art. As an example of this method, what is called a current ON-OFF starting method is disclosed in the U.S. Pat. No. 3,696,278, in which AC power is supplied to a controllable thyristor rectifier, the DC output from the controllable thyristor rectifier is supplied to a thyristor inverter through a DC reactor, the output of the thyristor inverter is supplied to a brushless motor and the brushless motor is started by causing the Dc current from the controllable thyristor rectifier to be turned ON and OFF intermittently. In this method, the rectifier is adapted to function temporarily a power regenerator circuit at each time when commutation of the brushless motor is made, that is, its DC current is temporarily reduced to zero for every instant of commutation, during the starting period of the brushless motor. Therefore, the ratio of the period of zero current from the rectifier to the commutation period is increased with increasing speed of the brushless motor, and there are such disadvantages as reduction of motor torque with reduction of the average current supplied to the motor and increase of pulsations of torque due to the ON-OFF control of the DC current.